Encapsulated electrical inductive apparatus



I Dec. 15, 19 70 H S PECKHAM 3,548,357

ENCAPSULATED ELECTRICAL INDUCTIVE APPARATUS Filed May 13, 1969 2Sheeas-Sheet 1 FIG].

WITNESSES INVENTOR Rodney L. Peckhom K WW cim R. PECKHAM ENCAPSULATEDELECTRICAL INDUCTIVE APPARATUS Filed May '13, 1969 Dec. 15,1910

2 Sheets-Sheet 2 FIG.2.

3,548,357 ENCAPSULATED ELECTRICAL INDUC'HVE APPARATUS Rodney L. Peckham,Transfer, Pa., assignor to Westinghouse Electric Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Filed May 13, 1969, Ser.No. 824,093 Int. Cl. H01f 27/32 U.S. Cl. 336205 7 Claims ABSTRACT OF THEDISCLOSURE Encapsulated electrical apparatus wherein at least oneelectrical winding has a plurality of radially superposed layers ofconductor turns. The turn-to-turn insulation in at least certain of thelayers is at least partially provided by a discrete continuousinsulating filament, solid or tubular, which separates the con-ductorturns of those layers.

BACKGROUND OF THE INVENTION (1) Field of the invention The inventionrelates in general to electrical inductive apparatus, such astransformers, and more specifically to transformers of the type whichare encapsulated in a cast solid resinous insulation system.

(2) Description of the prior art Basic insulation levels (BIL) arereference levels expressed in impulse crest voltage with a standard wavenot longer than l /2 40 microseconds. The rated withstand voltage is thecrest value of the impulse wave that the apparatus will withstandwithout disruptive discharge. The crest value of the rated Withstandcurve is the same as the BIL.

When electrical transformers are designed with BILs starting at about 60kv., it has been customary to utilize oil filled construction because ofthe excellent insulating properties of oil. Dry type transformers havebeen constructed to meet higher BILs, but they have required specialparts and constructions which differ from established manufacturingtechniques for dry type transformers, and thus have made themunattractive from a cost viewpoint. Therefore, it would be desirable tobe able to increase the BIL of dry'type transformers when the needarises, without special parts, special machines and/or manufacturingtechniques, which would make the dry type transformer of higher BILratings more competitive with similarly rated oil filled transformers.

SUMMARY OF THE INVENTION Briefly, the present invention is a new andimproved dry type transformer of the encapsulated or potted type, inwhich the BIL is increased while utilizing the same manufacturingapparatus and techniques which are used for conventional dry typetransformers. Further, the new and improved dry type transformer doesnot require the manufacture of special insulating shapes or members, orother specially designed or manufactured components of the type whichwould unduly increase the manufacturing cost.

The critical turn-to-turn insulation of the high voltage winding of thetransformer, at least in the layers of conductor turns immediatelyadjacent the high voltage terminal, is provided by one or morecontinuous, discrete insullating filaments or strands, either solid, ortubular, such as insulating sleeving, disposed between the conductorturns. The discrete continuous insulation is selected to have about thesame diameter as the wire of which the conductor turns are formed, withthe number of discrete filaments disposed between the adjacent conductorturns depending upon the electrical strength of the insulating "UnitedStates Patent 3,548,357 Patented Dec. 15, 1970 strands and theparticular BIL required. Insulating sleeving or tubing is readilyavailable, and it may be wound on the coil form at the same time theconductor turns are wound, similar to Winding a plurality of conductorstogether at the same time. Therefore, the same winding machines andtechniques may be used to achieve the required barrier dimension betweenconductor turns, as are presently used to manufacture conventional drytype trans formers.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of theinvention will become more apparent when considered in view of thefollowing detailed description and drawings, in which:

FIG. 1 is a perspective view, partially cut away, of a dry typeencapsulated transformer which may utilize the teachings of theinvention;

FIG. 2 is a cross sectional view of the winding assembly of thetransformer of FIG. 1, taken along a plane which cuts vertically throughthe assembly along a line between arrows II-II, illustrating the highvoltage winding constructed according to an embodiment of the invention;and

FIG. 3 is a fragmentary cross-sectional view of a high voltage windingconstructed according to other embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, andFIG. 1 in particular, there is illustrated a dry type encapsulatedtransformer 10, of the type which may utilize the teachings of theinvention.

In general, transformer 10 includes a magnetic corewinding assembly 12embedded in a cast solid insulation system 14, with the potted magneticcore-winding assembly and solid insulation being disposed in a suitablecasing or housing 16.

The magnetic core-winding assembly 12 comprises a magnetic corestructure 18, which in this instance comprises first and second magneticcores 20 and 22, respec tively, disposed in side-by-side relation, withtheir adjacent portions providing a winding leg about which a windingassembly 24 is disposed. As illustrated, each of the magnetic cores 22and 24 may be formed of a pair of wound C-cores, such as C-cores 26 and28, which are held in assembled relation by banding means 30 to providethe first magnetic core 20. Each of the C-cores have a plurality ofnested metallic laminations, such as laminations 32, which are bondedtogether to provide a coherent structure.

The winding assembly 24 includes concentrically disposed high and lowvoltage windings, with the high voltage winding having electrical leads34 and 36, and the low voltage winding having electrical leads 38 and40. Transformer 10 may have additional concentrically disposed windings,as required by specific applications.

The solid insulation system 14 may be of any suitable type, such as afilled epoxy resin. The filler material may be finely divided silica,such as sand, or any other inorganic non-friable material. Suitableresin systems for encapsulating dry type transformers are disclosed inUS. Pat. 3,030,597, which is assigned to the same assignee as thepresent application.

The magnetic core-winding assembly may be positioned Within the casing16 by a bracket member 42 which is welded to the inside of the casing.The bracket member 42 may also be used to maintain the potted magneticcore-winding assembly within the casing 16, by virtue of integral wingedprojections 44.

FIG. 2 is a cross-sectional view of the winding assembly 24 shown inFIG. 1, taken on a vertical plane through the assembly along a 'linebetween arrows IIII. Wind- 3 ing assembly 24 includes high and lowvoltage windings 50 and 52, respectively, constructed according to theteachings of the invention, to increase the BIL of the transformer to 60kv., or even higher, such as 75 or 95 kv.

The low voltage coil 52 is formed of metallic strip or foil, having aplurality of radially superposed insulated conductor turns, showngenerally at 54, which are wound on a mandrel or coil having a centerline 51. Since there is only one conductor turn per layer in a foil orstrip wound coil, the BIL of the low voltage coil 52 may be increased byincreasing the thickness of the insulation between adjacent conductorturns. Conservatively, normal insulation thickness between the turns maybe doubled, and should not be less than about 1.5 mils. The insulationbetween conductor turns may be provided by a coating of insulation, suchas enamel, disposed on one or both sides of the foil, or by a separatestrip or sheet of insulation which is wound with the metallic foil orstrip, such as a polyester film. The thickness of the insulation betweenthe conductor turns 54 of the low voltage winding 52 is selected toprotect the low voltage winding from high voltages which may becapacitively coupled into the low voltage winding from the high voltagewinding 50 during a surge condition.

The high-low insulation 56 is also increased in thickness by utilizing aplurality of layers of an impervious film, such as a polyester film.Since a polyester film has a withstand rating of about 4000 volts permil, a conservative safety factor of :1 would dictate about 150 mils ofpolyester film for a BIL of 60 kv.

The thickness of the solid insulation 14 adjacent the ends of the highand low voltage coils 50 and 52 is also increased, to about one inch fora 60 kv. BIL, and an additional 50 mils of insulation, such as apolyester film, is added between the ends of the winding assemblyadjacent the yoke portion of the magnetic core structure Theconstruction of the high voltage coil is especially critical when BILsof kv. and higher are desired, as a surge voltage applied to the highvoltage terminal doesnt distribute itself across the windinginductively, but according to the capacitive structure of the winding.Thus, instead of a uniform distribution of the surge voltage across thewinding, it is very non-linear between layers across the winding,between turns of the layers, and from the high voltage winding to thelow voltage winding and magnetic core, with the non-linearity being suchthat the electrical stress is concentrated at the portion of the windingconnected to the line terminal to which the surge is applied. Thus, thestress concentration is extremely high between the end turns of thelayers of turns connected to the line terminal, and between the endturns of the first two layers of turns, with most surge failureoccurring at these locations.

The present invention teaches how the high voltage coil 50 may beconstructed to BILs of 60 kv., 75 kv. and 95 kv., as required for ratedsystem voltages of 2.5 kv. to 15 kv., without requiring special shapesof insulating members or special manufacturing or process steps. Thusthe dry type encapsulated transformer may be constructed such that it iscompetitive costwise with oil filled types, for similar rated voltagesand BIL ratings.

More specifically, high voltage winding or coil 50 is formed of a wiretype conductor 59, such as copper, which is wound about the same centerline 51 as the low voltage coil 52, to form a plurality of layers, suchas layers 60, 62, 64, 66 and 68, of conductor turns, such as conductorturns 70 in layer 60. The conductor turns of the first layer 60 progressaxially from the first end of the winding assembly 24 to the second end,and then the next layer is radially superposed over the first layer,with its conductor turns progressing axially from the second to thefirst ends of the winding assembly. The Winding progresses in thismanner until all of the required layers of turns are provided.

The conductor 59 of which the conductor turns 70 are wound may have alayer or coating 61 of insulation disposed thereon, such as aninsulating enamel.

The critical turn-to-turn insulation is provided by a discretecontinuous filament or strand of insulating material, which in theembodiment of the invention shown in FIG. 2 is a length of insulatingsleeving or tubing 80, having an outside diameter which is substantiallythe same as the diameter of the wire conductor 59 with its layer orcoating 61 of insulation disposed thereon. Preferably, the diameter ofthe insulating tubing should be the same as, or slightly greater thanthe diameter of the insulated conductor.

Insulating tubing or sleeving 80 is readily available commercially, andit may be wound side-by-side with the insulated conductor 59, similar towinding two conductors side-by-side. The material of which theinsulating sleeving 80 is formed should be selected such that theelectrical strength presented by twice the wall thickness is higher thanthe BIL required. For example, a length of polyester tubing having awall thickness of 10 mils will provide a 20-mil barrier turn-to-turn. Ifa solid strand of insulation is used, the dimension of the strand fromconductor turn to conductor turn should provide the electrical strengthrequired. If the conductor of which the conductor turns is formed isrectangular, such as metallic strap, the insulating filament or strandmay also be substantially rectangular in shape, if desired.

The thickness of the layer insulation is increased to that required bythe particular BIL specified, with the layer insulation being graded, ifdesired, as shown in FIG. 2. In other words, since the greatestelectrical stress appears between the first few layers adjacent the lineterminal, more layers of insulation may be used between the first fewlayers than between the remaining layers of the winding. The dimensionW1 between the centers of the conductors of layers 60 and 62, andbetween the centers of the conductors of layers 62 and 64, is greaterthan the dimension W2 between the centers of the conductors of layers 64and 66, and layers 66 and 68, due to the grading of the layerinsulation. For example, in a transformer constructed according to theteachings of the invention for a BIL of 60 kv., 20 mils of polyesterfilm was provided between adjacent layers for the first four layers, andthen the layer insulation was reduced to ten mils for the remaininglayers.

FIG. 3 is a fragmentary view, in section, of a high voltage winding fora dry type encapsulated transformer, which is constructed according toother embodiments of the invention. FIG. 3 illustrates layers 102 and104, each of which have a plurality of conductor turns, such asconductor turns 106 in layer 102, which are formed of a conductor 108.In this embodiment, conductor 108 is bare, i.e., it does not have anintegral coating or wrapping of electrical insulation thereon, as theturn-toturn and layer insulation is completely provided by otherinsulating materials. Further. as illustrated in FIG. 3, each conductorturn may be separated by a plurality of discrete insulating strands orfilaments, with two strands 110 and 112 being illustrated. The number ofdiscrete strands disposed between adjacent conductor turns will dependupon the electrical strength of the insulation used, the -BILrequirements of the transformer, and the dimensions of the insulatingstrands. Still further, the insulating strands 110 and 112 areillustrated as being solid, instead of tubular in cross-sectionalconfiguration. Solid insulating strands are preferable over the tubulartype because for a given outer diameter the solid strand has a greaterelectrical strength than a tubular strand. The final choice, however,may be dictated by the fact that the tubular strands are more readilyavailable commercially than solid filaments of electrical insulationhaving the required dimensions.

In addition to grading the layer insulation, it would also be possibleto grade the turn-to-turn insulation by using more discrete filaments orstrands between the turns of the layers which are immediately adjacentthe line terminal, than between the turns in the remaining layers.

The start and finish leads of the high voltage winding 50 of transformershown in FIG. 2, are insulated with suitable insulating sleeving, andalthough it is not shown in the figure, the leads may be terminated inbushing members which may be cast into the solid insulation system 14.

An outer wrap of insulation is disposed about high voltage winding 50,such as about 150 mils of polyester film for a BIL of 60 kv.

After the winding assembly 24 is completed, the C-cores may be assembledabout the winding assembly, and the connections from the high voltagewinding 50 may be made to the bushing members, if used, and then thecomplete assembly is disposed within casing 16, after first invertingthe casing from the position shown. The magnetic core-winding assembly12 is oriented within the casing 16 with the bracket 42, which insuresthat the assembly will maintain the proper position relative to thecasing while the magnetic core-winding assembly is being encapsulated.The casing 16 and magnetic core-winding assembly is then heated to apredetermined temperature, such as 135 C., preparatory to receiving theliquid casting resin, and then the casting resin is introduced into thecasing to a predetermined level. A finely divided filler material, suchas sand, is then introduced into the casing until the resin level risesabove the top of the magnetic core assembly. The casing may be vibratedwhile the resin and sand are introduced, to assure complete impregnationof the core-winding assembly with the liquid resin, and uniformdispersion of the filler material through the liquid resin.

In summary, there has been disclosed a new and improved dry typeencapsulated transformer which may be constructed for BILs of 60, 75 and95 kv., without requiring special assemblies or members to bemanufactured, which would otherwise make the dry type transformeruncompetitive costwise with oil filled transformers. The increasedinsulating clearances from the high voltage winding to the magnetic coreand to the low voltage winding, are provided by additional layers ofsolid insulating films, the layer-to-layer insulation thickness in thehigh voltage winding is provided by additional layers of solidinsulating films, and the critical turn-to-turn insulation in the highvoltage winding is provided by discrete lengths of insulating material,either tubular or solid, which separate adjacent conductor turns in eachlayer of turns. Thus, the BIL of the dry type encapsulated transformermay be increased for approximately the cost of the additional insulationrequired, with the additional insulation being of the same general typewhich is already used in the dry type transformer, such as impervioussolid insulating films and insulating sleeving or tubing.

Since numerous changes may be made in the above described apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. An electrical transformer comprising magnetic core means having awinding leg,

high and low voltage windings disposed about the winding leg of saidmagnetic core means, to form a magnetic core-winding assembly, said lowvoltage winding including a plurality of radially superposed, in-

. sulated conductor turns formed of electrically conductive stripmaterial,

said high voltage winding including at least one electrical conductorwound to provide a plurality of radially superposed layers of conductorturns, insulating means disposed between the adjacent layers ofconductor turns, and at least one discrete, continuous insulatingfilament, the conductor turns of at least certain of the layers beingaxially separated by said at least one insulating filament, to provideturn-toturn insulation,

and solid insulating means, said solid insulating means capsulating saidmagnetic core-winding assembly.

2. The electrical transformer of claim 1 wherein the insulating filamenthas a tubular cross-section.

3. The electrical transformer of claim 1 wherein the insulating filamenthas a solid cross-section.

'4. The electrical transformer of claim 1 wherein the electricalconductor has a coating of electrical insulation disposed thereon.

5. The electrical transformer of claim 1 wherein the electricalconductor is free of electrical insulation, with the turn-to-turninsulation being completely provided by the insulating filament.

6. The electrical transformer of claim 1 wherein the radial thicknessdimension of the insulating means disposed between the layers ofconductor turns is graded, with the radial thickness dimension of theinsulation between certain layers exceeding that of the insulationdisposed between other layers.

7. The electrical transformer of claim 1 wherein the conductor turns ofat least certain of the layers are axially separated by a plurality ofcontinuous insulating filaments.

References Cited UNITED STATES PATENTS 685,470 10/1901 Heany 336-2071,816,680 7/1931 Kurath 336-205 ELLIOT A. GOLDBERG, Primary ExaminerU.S. Cl. X.R. 336-97, 207

